(a) Field of the Invention
This invention relates to repair materials for use in vessels for the containment of molten metals. These materials find use in, for example, the repair of blast furnace hearth linings and furnace stacks.
(b) Description of the Related Art
Blast furnaces are used to process iron ore into iron and cast the resulting product from the blast furnace in a molten form. The interior surfaces of blast furnaces are lined with refractory materials to contain the molten metal. Since blast furnaces are large, complex structures and very costly to rebuild, it is economically beneficial to increase the operating life of a given blast furnace as much as possible. Increased operating life leads to an increased need for interim repairs of the furnace refractory lining. An increase in time and resources devoted to the repair process reduces the efficiency of the blast furnace. Consequently, there is a need to reduce the time required, expense and complexity of the procedure used to repair the furnace lining.
The lining of the blast furnace hearth has carbon-based (e.g., carbon brick) or graphite-based materials on the wall combined with different types of cooling systems. A carbon-based grout may be used between the cooling system and the wall. A highly conductive material, such as carbon, is needed to transfer heat from the wall to the cooling system. The lining must also exhibit low permeability, high density, high strength, and high resistance to chemical attack.
In use, the lining is subjected to extremes of temperature, and must resist the materials with which it comes into contact. Because lining wear is uneven, certain parts of the lining may need repair before the lining as a whole needs replacement. The repair of a blast furnace hearth may take place in conjunction with a shutdown for stack shotcrete repairs. This typically occurs in about 18 to 24 month intervals. Total relines are very rare in current blast furnace operation, and might take place every 20 to 30 years. A material may be projected against the carbon-based or graphite-based material lining the interior of the vessel. This bonding material must be capable of bonding with carbon-based or graphite-based material, and must have chemically-resistive and physical-resistive properties similar to those of the carbon-based or graphite-based material on which it will be supported.
The hearth lining material must resist chemical attack by lead, zinc, iron and slag at the bottom of the hearth, and must resist physical degradation resulting from extreme conditions. Hearth temperatures may range from 2500° F. to 3000° F. (1371-1648° C.). The hearth lining material must also resist mechanical attack. Mechanical erosion is produced by moving and recirculating molten iron, and by molten iron draining out of the furnace. Additionally, mechanical erosion is increased by the ferrostatic pressure due to the volume of the vessel and high density of the iron above the hearth.
Certain known hearth lining materials contain different types of refractory aggregate, calcium aluminate cement, and other materials to yield a shotcretable material. A shotcretable material is mixed with water to a consistency that can be pumped through a concrete pump, and then sprayed by injecting air and an accelerator through a nozzle, to form a monolithic lining without the need for forms.
Another known blast furnace hearth lining repair formulation has been described as acting as an “artificial skull” to protect the damaged hearth. The application procedure included cleaning the hearth pneumatically, spraying a surfactant onto the hearth wall brick, and then shotcreting the walls with a silicon carbide (SiC) containing shotcrete mix. Shotcrete mix must have a particle size distribution permitting it to be pumped by a concrete pump. This application procedure has the disadvantages inherent to shotcreting, such as requiring large and expensive equipment and involving a long setup time, the requirement of an extra step of spraying on a surfactant in order for the material to adhere to the carbon brick, and the requirement that the shotcreting mix must have a particle size distribution permitting it to be pumped by a concrete pump.
Devices for dry pneumatically gunning, such as a Reed LOVA gun, Allentown N-1 gun, Piccola gun, etc. have been used to project refractory repair materials against the interior surfaces of the blast furnace. Previously known gunning procedures utilize the standard cooling down procedure followed for a particular blast furnace for a stack job and hearth repair. The side walls of the furnace are then pneumatically cleaned and the hearth repair material is gunned onto the walls. Ramping up the furnace may be accomplished by starting at about 70° F. (21° C.), then heating the material to 350° F. (177° C.), and maintaining the furnace at 350° F. (177° C.) for 8 hours. The furnace is then ramped to 600° F. (316° C.) over a period of 4 hours. Finally, the furnace is soaked at 600° F. (316° C.) for 12 hours. At this point the furnace is ready to start back up again.